TRANSFORMATION OF HELIOTROPIO ANIMALS 289 
2. There are photokinetic animals which react more 
rapidly to changes in the intensity of the light than do 
Planarians. I noticed this form of reaction at Naples in 
certain Annelids living in tubes; for example, Serpula 
uncinata. The gills of the animals are often exposed to the 
light. When the hand is moved between the animals and 
the source of light, they quickly draw back into their tubes 
as soon as they are struck by the shadow. In order to see 
whether positive and negative changes in the intensity of 
the light had the same effect, I made the following experi- 
ment: A glass aquarium which was closed by a glass cover 
was set upon an isolated table about 2 m. distant from the 
window. When I closed the shutters rapidly, the worms 
quickly withdrew into their tubes, much as does a snail when 
touched suddenly. The shutters did not close absolutely, 
and it was always light enough in the room to observe the 
animals. After some time the animals would again stretch 
out their gills. When I now suddenly opened the shutters 
quickly, the animals did not react. Even when the animals 
had withdrawn into their tubes, an increase in the intensity 
of the light did not again bring them out. It is therefore 
only the decrease in the intensity of the light which acts as 
a stimulus upon the animals. One notices, however, that 
these reactions cannot always be relied upon. Andrews has 
noticed such reactions also in Annelids whose gills are free 
from eyes or eye-like organs.’ 
VI. ON SOME PHYSIOLOGICAL CONDITIONS WHICH DETER- 
MINE THE DEPTH-DISTRIBUTION AND DEPTH-MIGRATION 
OF MARINE ANIMALS 
1. Investigations concerning the depth-distribution of 
marine animals seem to show that we meet with a consider- 
able amount of animal life only in two regions of the sea— 
1E, A. ANDREWS, Journal of Morphoiogy, Vol. V (1891). 
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